The invention pertains to frequency domain automatic equalization for electrical signals used in transmission of information.
Ideally, it is desirable to transmit electrical signals such that no interference occurs between successive symbols. In practice, however, transmission channels are bandlimited and intersymbol interference is controlled utilizing clocked systems with equalization conventionally performed in the time domain.
Most conventional automatic equalizers operate in a feedback mode so that the effects of changes in the equalizer transfer function are monitored and used to produce further changes in the transfer function to obtain the best output signals. In such systems, the measurements of the output signal are made in the time domain. Typically, the transfer function may be constructed in the time domain by adjusting the tap gains of a tapped delay line during an initial training period prior to actual meassage transmission. Examples of such systems are shown in U.S. Pat. Nos. 3,375,473 and 3,292,110.
Frequency domain equalization utilizing time domain adjustments are shown, for example, in the U.S. Pat. No. 3,614,673 issued to George Su Kang. Kang utilizes frequency domain measurement and calculations to produce the time domain impulse response of a transversal filter. The impulse response of the transversal filter is applied to set the weights of the transversal filter.
Other approaches to frequency domain equalization require transmission of the discrete Fourier transform of the source signal and require the use of complex analogue circuitry in obtaining an approximation to the desired equalization. See, for example, Weinstein and Ebert, "Transmission by Frequency -- Division Multiplexing Using the Discrete Fourier Transform", IEEE Transactions on Communication Technology, Vol. COM-19, No. 5, October 1971, pp. 628-634.
Mean square error techniques in various types of equalizers and filters are described, for example, in U.S. Pat. Nos. 3,763,359; 3,403,340; 3,889,108 and 3,657,669.